Adjustable speed drives are commonly used to drive electrical loads such as electric motors, pumps, and other cyclical equipment. Such drives typically include an inverter that converts power supplied by a main DC power bus to multi-phase current as appropriate for the load being driven. Adjustable speed drives also typically include a capacitor-resistor network designed to absorb variations in the main DC power bus and to discharge excess power such as during shut down of the drive. The resistors in the capacitor-resistor network are also typically used as a voltage divider (a passive voltage balancing system) to attempt to balance the DC voltage across the capacitors.
The capacitors in the capacitor-resistor network, like all capacitors, are prone to deterioration with time and usage. Deterioration during normal operation is generally gradual and uneven between the various capacitors, and so it may be expected that the passive voltage balancing system will slowly drift away from optimal performance. Deterioration may be greatly accelerated by an over-temperature, over-current, or over-voltage environment, potentially causing premature and unexpected failure of the capacitors. The failure of one of the capacitors in the network may trigger the failure of another of the capacitors due to the sudden over-voltages that may be experienced. This could potentially cause damage to the drive and/or to the load, and/or it may trigger a power fuse to open. It is therefore desirable to maintain the integrity of the capacitors for proper and safe operation of the adjustable speed drive.